1. Field of the Invention
The present invention relates to an inkjet printing apparatus and, more particularly, to a printing apparatus using an inkjet printing method in which images are formed by forming ink dots on a printing medium.
2. Description of the Related Art
An inkjet printing method is a method of printing in which images are formed by ejecting ink in a single color or a plurality of colors prepared for color printing onto various printing media such as paper, cloth, unwoven fabric, and plastic films for OHPs.
Inkjet printing apparatus employing this method include a so-called serial type inkjet printing apparatus having a carriage on which a printing unit (printing head) is mounted and which performs main scanning in a predetermined direction on a printing medium, a transport unit for transporting the printing medium in a direction different from the main scanning direction (sub-scanning), and a control unit for controlling those units. Ink is ejected from a plurality of ink ejecting openings provided on the printing head while performing serial scanning of the printing head in the main scanning direction. The printing head is transported a predetermined distance between serial scannings (e.g., a printing width that is achieved by one main scan and that is equivalent to the range in which the ejection openings are arranged). Thus, printing is sequentially performed on the printing medium.
Methods of supplying ink to a printing head used in an inkjet printing apparatus of this type include a method in which an ink tank is integrally or separably mounted on a printing head that is mounted on a carriage to be moved back and forth (main scanning) to supply ink to the same and a method in which an ink tank is provided separately from a printing head mounted on a carriage and is fixedly installed in a position in the printing apparatus other than the position of the head and in which ink is supplied by connecting the ink tank and the printing head through a flexible tube. Other methods include a method in which a supply system is configured such that ink is always or continuously supplied from an ink tank to a printing head in an amount in accordance with the amount of ejected ink and a method in which a printing head is provided with a reservoir section for reserving or storing a predetermined amount of ink (an amount to serve a sheet of printing medium, for example) and in which a supply system is configured such that ink is supplied at appropriate timing or intermittently to the reserving section from an ink supply source having a relatively great capacity.
Recording methods called full-multi or line types are also known which utilize a printing head having a multiplicity of nozzles arranged in a range that corresponds to the entire width of a printing medium and in which printing is performed while transporting the printing medium relative to the printing head in the longitudinal direction thereof (sub-scanning direction). In the case of such a full-multi type apparatus, there is no mechanism for scanning a carriage, unlike a serial type; an ink tank is fixedly mounted in another part of the apparatus.
Among such inkjet printing methods, the so-called drop-on-demand method in which printing is performed by ejecting ink directly onto a printing medium in accordance with printing signals is widely used as an easy and inexpensive method of printing.
The most widely known inkjet printing apparatus as described above are types in which an ink tank is provided in the form of a cartridge and in which the ink is replenished when used up by replacing the cartridge with a new one (cartridge replacement types).
There are various configurations for an ink tank as described above including a configuration in which an absorber in the form of a sponge or fabric is contained in the tank to fill the interior of the same and in which the absorbing element is impregnated with ink utilizing a capillary force thereof to hold the ink, a configuration which includes a section for containing such an absorber and another containing section for reserving ink as it is, and a configuration in which a bag is provided in a tank to contain only ink therein and in which a negative pressure is generated with a spring provided between the bag and an outer wall of the ink tank.
Inks used in inkjet printing apparatus are generally categorized into pigment type inks and dye type inks. Depending on purposes of use, some inkjet printing apparatus use dye type inks only; some inkjet printing apparatus use pigment type inks only; and some inkjet printing apparatus use both of dye type and pigment type inks. In some of inkjet printing apparatus that use both of dye type and pigment type inks, a pigment type black ink is used for printing of images that primarily involve black such as texts, and dye type color inks are used for printing of images that are primarily in colors such as those provided by a digital camera.
In using an inkjet printing apparatus, a user buys a new cartridge when ink runs out to replace the cartridge that has been used up as described above. Normally, such cartridges travel various routes of physical distribution for various physical distribution periods before they reach users. Changes can occur in the physical and chemical properties of inks in cartridges that have traveled various routes of physical distribution for various physical distribution periods. Specifically, components of an ink can be combined with each other to become particles having greater diameters or settlings in some occasions. Further, some types of ink easily undergo such changes in physical and chemical properties.
When such an ink including particles having greater diameters or settlings is supplied to a printing head, the ink can clog ink ejection openings and liquid passages in communication therewith to result in an ejection failure. In the case of a printing head in which a heater is used to generate thermal energy as energy for ink ejection, such an ink can cause problems such as kogation or burning which can damage the printing head.
In the case of inkjet printing apparatus for industrial purposes that perform printing on relatively large printing media as those used in the fields of textile printing and commercial printing, a cartridge can be continuously used without becoming a waste when it has a configuration that allows the ink to be replenished properly in accordance with the progress of ink consumption. However, settlings can be deposited on the bottom while the cartridge is used for a long time while being replenished with ink repeatedly.
In order to prevent problems attributable to the use of an ink whose condition has changed in a cartridge, there are proposals on an inkjet printing apparatus having a transport unit aimed at agitation of an ink separate from a transport unit for replenishing an ink chamber with the ink and an inkjet printing apparatus in which a single pump mechanism is used for both of replenishment and agitation of an ink. The purpose is to agitate an ink whose condition has changed in a cartridge, thereby stirring settlings into the ink again to achieve a uniform condition.
In some inkjet printing apparatus having no agitating unit, changes of a certain degree in ink condition on the way of physical distribution and during a physical distribution period are assumed; the portion of the ink whose condition is assumed to have changed is discharged when the cartridge is replaced with new one using a pump unit that is provided in the inkjet printing apparatus for the purpose of charging ink; and printing is performed using the remaining ink.
However, such inkjet printing apparatus have the following problems to be solved.
An ink supply unit and an agitating unit provided in an inkjet printing apparatus create problems such as an increase in the size of a main body of the apparatus and an increase in the manufacturing cost of the apparatus main body, and those units are therefore unsuitable for inkjet printing apparatus to be used in offices or homes.
Even when an ink supply unit serves also as an agitating unit, sufficient agitating performance cannot be achieved in a configuration in which an ink is held in the interior of an ink tank as a whole with a capillary force of an absorber in the form of a sponge or fabric.
Assuming that an ink discharging condition is determined for a long physical distribution period, when a user actually installs and uses an ink tank that has been distributed for a short time, the ink will be discharged in the same amount as that discharged from an ink tank that takes a long time to be distributed. This results in an indiscriminate increase in the running cost.
Assuming that an ink discharging condition is determined for a short physical distribution period, when a user actually installs and uses an ink tank that has been distributed for a long time, the ink will be discharged only in the same amount as that discharged from an ink tank that takes a short time to be distributed. There will be an ink residue whose condition has changed in the ink chamber of the printing head, which can result in problems such as an ejection failure or disabled ejection in nozzles that are not in use.
Such an ejection failure is unlikely to occur at nozzles that are continuously used and is likely to occur at nozzles that are intermittently used (nozzles associated with inks that are not used for forming a certain image).
It is considered that the failure is likely to occur at the intermittently used nozzles because of a difference between quantities of heat accumulated at the continuously used nozzles and the intermittently used nozzles.